The invention uses various materials which are electrically either conductive, insulating or semiconducting, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor". One of the materials used is silicon, which is used as either single crystal silicon or as polycrystalline silicon material, referred to as polysilicon or "poly" in this disclosure.
In many transistor circuits, it is necessary to form both n-channel transistors and p-channel transistors, in a complimentary metal oxide semiconductor (CMOS) circuit.
In the fabrication of a CMOS circuit, an n-well is first formed by masking a semiconductor wafer and implanting an impurity. The impurity is intended to infiltrate to a sufficient depth to define the n-well, as an n- material of sufficient depth to permit the functioning of the p-channel transistor circuit.
A typical dopant used is phosphorus which has a characteristic of being fairly easy to control in depth and distribution.
After forming the n-wells, a thin layer of oxide, in the range of 250 A, is grown on the wafer to form an initial oxide layer. The nitride is deposited and the wafer is then masked again for the purpose of defining active areas (AA). The mask is stripped and a photomask is then applied over n-wells. A boron implant is applied to the p-type area, with the boron functioning as a channel-stop implant. The boron does not penetrate the nitride and, therefore, the nitrided areas do not receive the channel-stop implant. The photoresist is then stripped and fieldox (field oxide) is grown in areas which are not layered with nitride. The wafer is then masked again in order to shield the n-well, and a punch-through implant is applied to the n-type transistors, using boron dopant.
A blanket implant of some boron is applied through the field area prior to growth of field oxide, in order to raise n-channel field transistor threshold voltage V.sub.T. P-channel field transistor V.sub.T may be degraded by this blanket boron implant, but will be improved by the use of arsenic in n-wells. N-channel field transistor V.sub.T is further increased by the use of boron high energy punch-through implant.
The choice of dopants is made in accordance with the electrical effects of the dopant and the ease at which the dopant is implanted. Since the ease of implant affects the distribution of the dopant, this, of course, also affects the electrical characteristics of the device. As mentioned, phosphorus is a fairly easy material to implant in that it requires a relatively low energy to penetrate the silicon wafer to a desired depth. Other dopants, such as arsenic, require more energy and tend to concentrate at a certain level; this level is determined by the amount of energy used in implanting the dopant.
Phosphorus is said to have a high diffusion coefficient, meaning that phosphorus is relatively easy to diffuse into the wafer. Arsenic, on the other hand, has a lower diffusion coefficient. Applying more energy to diffuse arsenic results in the arsenic concentrating at a different level, a phenomenon which is referred to (at least here in Idaho) as the snowplow effect.
In the prior art, n-channel transistors are provided with a channel-stop implant by first photomasking over the p-channel areas, and then applying the implant. The implant is a boron implant which creates a p+ material under areas which eventually will be defined as field areas by growing fieldox. The mask is used to prevent the boron from penetrating the transistor areas and thereby shorting the p-channel transistors.
U.S. Pat. No. 4,839,301, to inventor Lee, entitled Blanket CMOS Channel Stop Implant Employing a Combination of n-Channel and p-Channel Punch-Through Implants discloses a CMOS transistor fabricated by forming the n-wells with both phosphorus and arsenic implants. The arsenic, with its lower diffusion coefficient, tends to concentrate near the top surface of the n-wells, with the phosphorus penetrating sufficiently to define the n-wells at the desired depth. The use of arsenic had improved p-channel short-channel effects, and had compensated low-dose boron from blanket boron field implant This resulted in the savings of one mask.
A boron channel-stop implant was later applied without masking over the n-wells. Therefore, the boron implant for the n- and p-channels caused the boron to also be diffused into the n-wells. Since the arsenic implant is concentrated near the surface, the arsenic impurities overcame the effects of the boron impurities, thereby causing the top surface of the n-wells to remain as n- material.
The arsenic dopant was adjusted to that which would allow the n-well to remain as n- material and avoid the short-channel effect. It was also necessary to optimize the blanket boron field implant so that the degradation of p-channel field transistor V.sub.T will not be greater than the improvement made by the arsenic dopant punch-through. The high energy boron implant, on the other hand, was determined by the n-channel device requirements and by the additional n-channel field transistor V.sub.T improvement requirements.
Prior art techniques use blanket channel stop implant by only introducing arsenic in the n-well. One prior art technique avoids the use of a channel stop and V.sub.T adjustment photomask. Arsenic implant is introduced to the n-well in order to produce a high surface concentration and to serve as a channel-stop for p-channel devices. A second implant is a deep boron implant which is selectively placed into the p type material. The use of an implant mask is avoided because an oxide layer is placed over the n-wells as an oxide mask. That technique is disclosed in Chen, et al., A Hiqh Performance Submicron CMOS Process With Self-Aligned Chan-Stop and Punch-Through Implants, IEDM Vol. 86, pp 256-59.
The process of U.S. Pat. No. 4,839,301 avoided the requirement of the oxide mask, while still using a blanket channel-stop implant. The n-wells were formed with both phosphorus and arsenic implants. An n-channel punch-through implant was used to improve the n-channel field transistor characteristics. Without the n-channel punch-through implant, the n-channel field transistor characteristics was thought to be unacceptably degraded. Also, without the n-channel punch-through implant to improve the n-channel field transistor V.sub.T, excessive blanket boron field implant dose was believed to be required. Since this would require more arsenic in the n-well to compensate for the boron, avoiding the punch-through implant was thought to inevitably degrade the p-channel transistor and speed.
From economical points of view, it is desirable to omit the number of mask steps used for fabricating integrated circuits. Therefore, any solution to the short channel effect should not result in substantially increasing masking; ideally it should result in a decrease in mask steps.